Recently, a number of physiologically active proteins are used or ready to be used as pharmaceutical drugs. Since these physiologically active proteins are all easily metabolized, decomposed or excreted when administered to the body of animals including human beings, their retention time in blood is short and their target directivity is low, thus posing a problem in that the protein is not accumulated in the affected part in a required amount for a required period of time.
Various attempts have been made with the aim of overcoming this problem. For example, as described in a report by Karte et al (Proc. Natl. Acad. Sci., USA, 84 (1987), pp.1487-1491), there is a method in which a physiologically active protein is chemically modified with polyethylene glycol.
However, all such chemical modification methods find difficulty in modifying a specified part of the protein or strictly controlling the degree of modification. For example, when a protein is modified with polyethylene glycol, polyethylene glycol is introduced mainly at the .epsilon.-position amino group of a lysine residue in the protein, but, since a protein molecule generally contains a plurality of lysine residues, polyethylene glycol may be incorporated or not incorporated into a plurality of lysine residues. As the result, the protein loses its inherent physiological activity in some cases or its quality as a pharmaceutical drug cannot be controlled easily.
For these reasons, great concern has been directed toward the development of a method which can modify a specified site of a physiologically active protein or can strictly control degree of the modification when the physiologically active protein is modified with a modification agent such as polyethylene glycol, a polysaccharide or the like.